1. Field of the Invention
The present invention relates to a ring-free zero-voltage switching technique for use in switching power converters and, more particularly to such a ring-free zero-voltage switching technique, which greatly reduces magnetic loss and switching loss during high-performance high-density switching process of the switching power converter.
2. Description of the Prior Art
In recent years, in order to follow the tendency of making electronic products smaller, the development of switching power converter technology is aimed at high frequency, high performance, and high density. Normally, the switching speed of power MOSFET is much faster than bipolar transistor. Therefore, power MOSFET is intensively used in switching power converter for use as a power switch. However, the energy accumulated in parasitic capacitance of power MOSFET will be used up by means of ohmic heating in the passage each time power MOSFET is electrically connected. The higher the switching frequency is the greater the loss will be. If this problem is not eliminated, power-switching converter cannot achieve high performance and high density switching operation.
Since the report of zero-voltage switching issued by C. P. Henze, H. C. Martin and D. W. Paraley on IEEE in 1988, several practical circuits have been disclosed to effectively eliminate connection loss of conventional power MOSFET. Exemplars of these prior techniques are outlined hereinafter:
(1) Forward Zero-Voltage Switching Power Converter:
The circuit shown in FIG. 1(A) is the embodiment of U.S. Pat. No. 383,594 designed by Bruce Wilkinson and filed in June 1989. This patent enables transformer to work in between positive and negative magnetic zones by means of controlling the circuit, therefore under same output power, relatively smaller transformer can be used. By means of this circuit design, Putrice R. Lethellier invented a first practical zero-voltage switching circuit. This circuit architecture, as shown in FIG. 1(b), is granted a U.S. Pat. No. 4,975,821 in October 1990. In order to achieve zero-voltage switching, transformer adopts loose cross-linking, and gap is provided in its magnetic core, so as to obtain the requisite magnetizing inductance and leakage inductance, for enabling magnetizing inductance and leakage inductance to form a L-C resonance circuit with parasitic capacitance Cs being connected in parallel to switch SW1, which enables switch SW1 to obtain zero-voltage switching condition at the moment SW2 is switched off. Similarly, the resonance of L-C resonance circuit enables switch SW2 to obtain zero-voltage switching condition at the moment SW1 is switched off, however since the existence of gap and leakage inductance in magnetic core of transformer would cause a significant magnetic loss, which resulting in abnormal production of heat and lowering of performance of transformer when the circuit achieving the zero-voltage switching condition.
FIGS. 2(a) and 2(b) show the circuits of U.S. Pat. No. 5,245,520 issued in September 1993, filed in October 1991 by Paul Imbertson. Circuit of FIG. 2(a) can be called as xe2x80x9cHalf bridge asymmetrical buck converterxe2x80x9d. Circuit of FIG. 2(b) can be called as xe2x80x9cFull bridge asymmetrical buck converterxe2x80x9d. Because these two circuits have been added with an auxiliary inductor La equivalent to transformer leakage inductance, the transformer eliminates the problem of abnormal heating when achieving zero-voltage switching. However, auxiliary inductor La and the stray capacitance at two ends of the primary winding of the transformer would produce a significant ringing. Because the ringing current oscillates between the inductor and the winding of the transformer, it produces induction heating to the magnetic core, and lowers down the performances thereof. Further, because the parasitic oscillation increases EMI noises to the secondary winding of the transformer, the reverse voltage rating of the secondary rectifier elements must be increased by at least 1.5 times. These are the drawbacks induced by the ringing on the windings of the transformer of Imbertson""s circuit. FIG. 3 shows the ringing on the primary winding and the secondary winding of the transformer of Imbertson""s circuit.
(2) Flyback Zero-Voltage Switching Power Converter:
FIGS. 4(a) and 4(b) show the circuits of U.S. Pat. No. 5,057,986 issued in October 1991 to Christopher P. Henze and Hubert C. Martin Jr. This disclosure eliminates the use of an auxiliary inductor La. In order to achieve zero-voltage switching, the gap of the transformer must be greatly increased, to let the peak-to-peak value of the primary magnetizing current of the transformer be greater than the load current to the primary side reflected by the secondary side. Similar to Putrice R. Lethellier""s patented invention, this design causes abnormal heating of the transformer. In order to eliminate abnormal heating, the size of the transformer must be greatly increased so as to improve the capability of dissipating heat of the transformer.
FIGS. 5(a) and 5(b) show the circuits of U.S. Pat. No. 5,402,329 issued in March 1995 to Wittenbreder, Jr. and Ernest H. Because these circuits use an auxiliary inductor, they can easily achieve zero-voltage switching without making the peak-to-peak value of the primary magnetizing current of the transformer to be greater than the load current reflected to the primary side by the secondary side. The auxiliary inductor can be a leakage inductance during cross-linking winding of the transformer, or an added inductor. No matter the auxiliary inductor exists in what kinds of type, this patent can not eliminate the problem of the side effect caused by ringing as encountered in Imbertson""s patent. FIG. 6 shows the ringing on the primary winding and the secondary winding of the transformer of the circuits designed by Wittenbreder, Jr. and Ernest H.
The present invention has been accomplished to provide a ring-free zero-voltage switching technique for use in switching power converters, which eliminates the occurrence of induction heating effect to the magnetic core caused by parasitic ringing during zero-voltage switching of power rectifier switches, preventing reverse voltage impact to rectifier components and eliminating EMI noises.
According to one embodiment of the present invention, the zero-voltage switching circuit with auxiliary inductor and balance capacitor of a conventional switching power converter is re-arranged, and at least one inductor current shorted diode is added to the zero-voltage switching circuit to suppress parasitic oscillation formed by the stray capacitance at the primary side of main transformer of the switching power converter, so as to prevent the auxiliary inductor and the main transformer from producing EMI noise, to effectively lower the reverse voltage rating requirement of secondary side rectification component, to greatly increase the working frequency and power density, and to let the switching power converter meet the requirements of international EMI regulations.
According to another embodiment of the present invention, the auxiliary inductance of the zero-voltage switching circuit is installed in the series circuit between the transformer and two power MOSFETs, and at least one diode is added to the circuit connection between the primary winding of the transformer and the added inductor, so that the diode works with the corresponding power MOSFET to short-circuit the electric current of the auxiliary inductor and to suppress the voltage of the stray capacitor at the primary side of the transformer upon the occurrence of ringing of the added inductor, preventing the stray capacitor from oscillation, so as to achieve high-performance high-density low-noise zero-voltage switching operation.
Furthermore, the ring-free switching technique of the present invention can be used in flyback, boost-forward, and boost-flyback switching power converters to prevent the occurrence of ringing, so as to effectively reduce power loss and lower the reverse voltage rating requirement of secondary side rectification component, to greatly increase the working frequency and power density, to eliminate EMI noises, to minimize the size of the heat sink required for dissipating heat energy from the power MOSFETS, and to let the switching power converters be applicable to the designs of mini electronic products.